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Author Topic: Why do power lines affect AM, but not FM, radio signals?  (Read 6130 times)

Offline chris

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Driving under power lines usually causes disturbances to AM radio reception, yet FM is unaffected. Why is this?


 

Online alancalverd

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Re: Why do power lines affect AM, but not FM, radio signals?
« Reply #1 on: 01/05/2015 10:22:28 »
Power lines emit electromagnetic radiation, not only at the line frequency (50 0r 60 Hz) but also whenever the line current changes due to load switching or noise injected by the load. In the case of grid power lines, which are supplying multiple randomly-switched loads, this amounts to a considerable emission at frequencies up to around 1 - 2 MHz.

Electromagnetic waves add by superposition. Thus the instantaneous amplitude of the signal received by your radio, particularly at low frequencies, is the sum of the desired signal plus local noise.

AM, as the name implies, transmits the audio signal (say 50 - 15,000 Hz) by modulating the amplitude of the carrier wave (around 0.1 to 10 MHz). Thus any spurious addition to that amplitude will appear as noise  impressed on the audio signal, and extreme noise can saturate the receiver input to the point that the audio signal cannot be heard at all.

FM, in contrast, transmits a cleaner audio signal (potentially 20 - 20,000 Hz) by varying the frequency of the carrier (generally 80 - 108 MHz). The noise from power lines is mostly at lower frequencies and in any case will only affect the amplitude, not the frequency, of the radio wave you are tuned to. Thus FM reception is much less prone to fading and interference than AM: you either have enough carrier amplitude to lock on and decode the audio, or you don't.

Curiously, the reason we continue to use AM for most air and sea radiotelephony is exactly this! Being susceptible to fading, we can use the same carrier frequency at well-separated locations without interfering with one another, and a powerful ground station can "stamp on" low-power air-to-air chatter to give urgent instructions to "all stations in the vicinity".  Flying or sailing under power lines is never a good idea anyway.
 

Offline PmbPhy

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Re: Why do power lines affect AM, but not FM, radio signals?
« Reply #2 on: 01/05/2015 10:27:17 »
Quote from: chris
Driving under power lines usually causes disturbances to AM radio reception, yet FM is unaffected. Why is this?
Because AM means Amplitude Modulation which means that the information in an AM radio wave is encoded in the amplitude of the EM wave. If you drive under power lines then the EM field of the power lines is added to the field of the AM radio signal thus modifying the amplitude and thus changing the information encoded in it. FM means Frequency Modulation which means that the information in an FM radio wave is encoded in the frequency. If you drive under power lines then the EM field of the power lines is added to the field of the FM radio signal but in this case the frequency is unaffected and as a result the information encoded in it is unaffected.
 

Offline PmbPhy

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Re: Why do power lines affect AM, but not FM, radio signals?
« Reply #3 on: 01/05/2015 10:28:27 »
Seems that Alan and I posted simultaneously.
 

Online alancalverd

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Re: Why do power lines affect AM, but not FM, radio signals?
« Reply #4 on: 01/05/2015 10:36:25 »
Great minds think alike. Fools seldom differ.
 

Offline Colin2B

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Re: Why do power lines affect AM, but not FM, radio signals?
« Reply #5 on: 01/05/2015 18:05:04 »
Great minds think alike. Fools seldom differ.
No comment!
 

Offline evan_au

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Re: Why do power lines affect AM, but not FM, radio signals?
« Reply #6 on: 02/05/2015 03:06:24 »
In addition to the comments above about the method of modulation, there is also the issue of the wavelength and interference patterns:
  • FM broadcasts typically use frequencies around 100MHz, which represents wavelengths around c/100x106Hz = 3x108/1x108 ≈ 3m. Because high-voltage power lines are typically 20m or more above the road, the much smaller wavelengths of FM stations can directly reach the area under the power lines.
  • AM typically uses frequencies around 1MHz, which represents wavelengths around 300m. The overhead power lines form a radio shield whose gaps are much smaller than the wavelength, so they block the AM signal much more effectively than they block FM signals.
  • Even if the radio signal is not entirely blocked, you can get interference patterns due to signal reflection from the power lines. For AM, this represents a fadeout which lasts perhaps 50-150m of roadway. For FM, the corresponding fadeouts last perhaps 0.5-1.5 meters of roadway ("picket fence" effect).
  • How the radio receiver deals with the interference also differs: AM radios do not change the volume depending on the signal level, so a fadeout means the audio disappears. However, the textbook FM receiver uses "infinite" amplification, so it doesn't matter whether the signal is reduced by a factor of 10 or 100, it still sounds the same (there might be a problem if the signal level were reduced by a factor of 1 million; in practical receivers, "infinite" < 10 million). 
     
Quote from: alancalverd
the reason we continue to use AM for most air and sea radiotelephony is ... we can use the same carrier frequency at well-separated locations without interfering with one another
Station separation depends a lot on the frequencies you use.
  • FM broadcast transmissions around 100MHz are pretty much line-of-sight. FM receivers exhibit a "capture effect", where they will lock onto the strongest signal, and ignore the weaker one. This allows you to pack a lot of terrestrial FM transmitters in adjacent towns on the same frequencies. As soon as drive over the transmitter's horizon, the signal gets rather erratic, switching the station to which it is locked every time you drive behind a hill. FM signals pass straight through the ionosphere into outer space.  (In an airplane, because the plane may be in sight of several transmitters at once, so it locks onto the strongest signal). 
  • Much of the broadcast-band AM signal around 1MHz propagates as a surface-wave, so it travels around the surface of the Earth, providing extended coverage for surface vehicles beyond the horizon; this means you need to change AM stations less often when traveling in your car than for FM (some modern radios will automatically switch to the most consistent strong station). This surface propagation works especially well over water. At night, you can sometimes get AM radio reflections from the ionosphere, resulting in quite long-range reception (hundreds of km, or thousands of km in the shortwave band, 3-30MHz).
  • The modulation format choice is somewhat independent of the frequency choice; by using AM at 40MHz or above, propagation is more line-of-sight, and you can pack stations closer together, while still allowing stations to "shout" in emergency situations. 
 

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Re: Why do power lines affect AM, but not FM, radio signals?
« Reply #6 on: 02/05/2015 03:06:24 »

 

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